[Numpy-discussion] C extension compiling question

[Jon Wright]

Dear Henry,

You need to call import_array() in initspam. See:
http://docs.scipy.org/doc/numpy-1.5.x/user/c-info.how-to-extend.html

HTH,

Jon


On 29/10/2010 15:11, Henry Gomersall wrote:
> I'm trying to get a really simple toy example for a numpy extension
> working (you may notice its based on the example in the numpy docs and
> the python extension docs). The code is given below.
>
> The problem I am having is running the module segfaults at any attempt
> to access&PyArray_Type (so, as presented, the segfault occurs at the
> call to PyArg_ParseTuple). I've been assuming that importing numpy into
> the calling python code implicitly provides the numpy libraries that are
> necessary for linking. I can only assume I'm missing something. Do i
> need some additional includes etc in my compile flags?
>
> I'm currently using the distutils.core module and not the distutils
> module that comes with numpy if that makes a difference.
>
> I'm actually pretty keen to document what I've learnt as a 5 minute
> how-to for writing basic numpy extensions, if there is demand for it. It
> doesn't seem to exist as a coherent whole at the moment.
>
> Thanks,
>
> Henry
>
> #include<Python.h>
> #include<numpy/arrayobject.h>
>
> PyMODINIT_FUNC
> initspam(void);
>
> static PyObject *
> spam_system(PyObject *self, PyObject *args);
>
> void
> worker_function(double *input, double *output, npy_intp size);
>
> static PyMethodDef SpamMethods[] = {
>      {"system",  spam_system, METH_VARARGS,
>       "Execute a shell command."},
>      {NULL, NULL, 0, NULL}        /* Sentinel */
> };
>
> PyMODINIT_FUNC
> initspam(void)
> {
>      (void) Py_InitModule("spam", SpamMethods);
> }
>
> static PyObject *
> spam_system(PyObject *self, PyObject *args)
> {
>      int n;
>      int ndims;
>
>      PyObject *inarg=NULL, *outarg=NULL;
>      PyObject *inarr=NULL, *outarr=NULL;
>
>      /* Both input and output need to be PyArray_Type type*/
>      if (!PyArg_ParseTuple(args, "O!O!",&PyArray_Type,&inarg,
> &PyArray_Type,&outarg));
>          return NULL;
>
>      inarr = PyArray_FROM_OTF(inarg, NPY_DOUBLE, NPY_IN_ARRAY);
>      if (inarr == NULL) goto fail;
>
>      outarr = PyArray_FROM_OTF(outarg, NPY_DOUBLE, NPY_OUT_ARRAY);
>      if (outarr == NULL) goto fail;
>
>      /* Check the shape of the two arrays is the same */
>      ndims = ((PyArrayObject *)outarr)->nd;
>      if (((PyArrayObject *)inarr)->nd != ndims)
>          goto fail;
>
>      /* ...still checking...*/
>      for (n=1; n<ndims; n++)
>      {
>          if (PyArray_DIM(inarr,n) != PyArray_DIM(outarr,n))
>              goto fail;
>      }
>
>      /* Now actually do something with the arrays */
>      worker_function((double *)PyArray_DATA(inarr),
>                          (double *)PyArray_DATA(outarr),
>                          PyArray_Size(inarr));
>
>      Py_DECREF(inarr);
>      Py_DECREF(outarr);
>      return Py_None;
>
> fail:
>      Py_XDECREF(inarr);
>      Py_XDECREF(outarr);
>      return NULL;
> }
>
> void
> worker_function(double *input, double *output, npy_intp size)
> {
>      npy_intp n;
>
>      for (n=0;n<size;n++)
>      {
>          output[n] = input[n] + 10;
>      }
>      return;
> }
>
>
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